The present invention relates generally to a multiple conveyor system and, more particularly, to a multiple conveyor system having at least two parallel, superimposed, movable conveyor frames.
A variable length conveyor system embodying at least two independent conveyor belts on separate superimposed conveyor frames is illustrated in my U.S. Pat. No. 3,598,224, the teachings of which are incorporated by reference herein. In this system, at least two conveyors are supported above an extendible boom of a conventional crane-type wheeled vehicle. A main conveyor is rigidly affixed to the main, immovable portion of the extensible boom and is equipped with an endless conveyor belt driven by a suitable motor. Positioned between the main conveyor and the extensible portion of the boom and rigidly affixed thereto is a second, separate conveyor frame containing a separate conveyor belt and drive motor. The lower extensible conveyor moves back and forth parallel to the main conveyor in response to the extension and retraction of the extendible boom. During this movement, at least a portion of the extendible conveyor is positioned below the discharge end of the main conveyor to receive materials such as concrete as they are discharged from the main conveyor. In this regard, the materials must fall substantially directly downward from the main conveyor to the extendible conveyor.
This conveyor system has some disadvantages which, while not interfering with the basic purposes of the system, result in a somewhat more expensive and bulkier system. For example, in the conventional conveyor system, each of the conveyors utilizes a separate motor and drive system. As a consequence, when the boom is fully extended, suitable wires and controls must be provided to extend from the crane control cab to the motor on the extensible conveyor frame now substantially extended outward. Further, when the boom is extended in this outward position, the motor weight of the extensible conveyor frame must be counteracted by the weight of the crane chassis to prevent the entire system from tipping forward. In other words, as a result of the motor for the extensible conveyor frame being positioned a substantial distance from the center of gravity of the entire crane system, outriggers or other suitable counterweights must be provided to maintain stability for the entire system. In addition, there is substantial cost and weight associated with the electric lines and hydraulic hoses that are necessary to power the motor for the extensible conveyor.
A further disadvantage of the prior art multiple conveyor system is that concrete, as it falls or is thrown from the main conveyor down onto the extensible conveyor, often has the tendency to bounce from the second conveyor belt. To prevent excessive loss of the concrete, scrapers and baffles have been positioned near the discharge point of the main conveyor. These baffles, etc., result in a perfect drop of the concrete between conveyors but, as in the case of the separate motor for the extensible conveyor, result in an increased mass at a point substantially removed from the center of gravity of the vehicle per se. Again this results in either lower capacity on the conveyors and/or suitable counterweights or outriggers on the vehicle.
Although single belt - single motor extendible conveyor systems are presently known, these systems are generally unacceptable for the transfer of semifluid materials, such as concrete. Such a system is shown in Brevet D'Invention No. 1,267,843 (delivery date July 13, 1960).
These systems are unacceptable because the conveyor belt must remain flat throughout the conveyor run. As such, a semifluid material would run off the edges of the belt. A flat conveyor belt is necessary to permit engagement and disengagement of the belt and the various supporting idler assemblies of the conveyor system during movement between a retracted and extended state.
Modification of such systems to permit semifluid material transfer is also prohibited. The presently known idler assemblies, utilized in concrete-carrying conveyor systems, do not permit belt engagement and disengagement without forfeiture of belt alignment capabilities.